Carbohydrates constitute the most abundant organic compounds on earth. However, much of this carbohydrate is sequestered in complex polymers including starch (the principle storage carbohydrate in seeds and grain), and a collection of carbohydrates and lignin known as lignocellulose. The main carbohydrate components of lignocellulose are cellulose, hemicellulose, and glucans. These complex polymers are often referred to collectively as lignocellulose. Cellulose is a linear polysaccharide composed of glucose residues linked by beta-1,4 bonds. The linear nature of the cellulose fibers, as well as the stoichiometry of the beta-linked glucose (relative to alpha) generates structures more prone to interstrand hydrogen bonding than the highly branched alpha-linked structures of starch. Thus, cellulose polymers are generally less soluble, and form more tightly bound fibers than the fibers found in starch. Hemicellulose is a complex polymer, and its composition often varies widely from organism to organism, and from one tissue type to another. In general, a main component of hemicellulose is beta-1,4-linked xylose, a five carbon sugar. However, this xylose is often branched as beta-1,3 linkages, and can be substituted with linkages to arabinose, galactose, mannose, glucuronic acid, or by esterification to acetic acid. Hemicellulose can also contain glucan, which is a general term for beta-linked six carbon sugars. The composition, nature of substitution, and degree of branching of hemicellulose is very different in dicot plants as compared to monocot plants. In dicots, hemicellulose is comprised mainly of xyloglucans that are 1,4-beta-linked glucose chains with 1,6-beta-linked xylosyl side chains. In monocots, including most grain crops, the principle components of hemicellulose are heteroxylans. These are primarily comprised of 1,4-beta-linked xylose backbone polymers with 1,3-beta linkages to arabinose, galactose and mannose as well as xylose modified by ester-linked acetic acids. Also present are branched beta glucans comprised of 1,3- and 1,4-beta-linked glucosyl chains. In monocots, cellulose, heteroxylans and beta glucans are present in roughly equal amounts, each comprising about 15-25% of the dry matter of cell walls.
The sequestration of such large amounts of carbohydrates in plant biomass provides a plentiful source of potential energy in the form of sugars, both five carbon and six carbon sugars that could be utilized for numerous industrial and agricultural processes. However, the enormous energy potential of these carbohydrates is currently under-utilized because the sugars are locked in complex polymers, and hence are not readily accessible for fermentation. Methods that generate sugars from plant biomass would provide plentiful, economically-competitive feedstocks for fermentation into chemicals, plastics, and fuels. Current processes to generate soluble sugars from lignocellulose are complex. A key step in the process is referred to as pretreatment. The aim of pretreatment is to increase the accessibility of cellulose to cellulose-degrading enzymes, such as the cellulase mixture derived from fermentation of the fungus Trichoderma reesei. Current pretreatment processes involve steeping lignocellulosic material such as corn stover in strong acids or bases under high temperatures and pressures. Such chemical pretreatments degrade hemicellulose and/or lignin components of lignocellulose to expose cellulose, but also create unwanted by-products such as acetic acid, furfural, hydroxymethyl furfural and gypsum. These products must be removed in additional processes to allow subsequent degradation of cellulose with enzymes or by a co-fermentation process known as simultaneous saccharification and fermentation (SSF). The conditions currently used for chemical pretreatments require expensive reaction vessels, and are energy intensive. Chemical pretreatment occurring at high temperatures and extreme pH conditions (for example 160° C. and 1.1% sulfuric acid at 12 atm. pressure) are not compatible with known cellulose-degrading enzymes. Further, these reactions produce compounds that must be removed before fermentation can proceed. As a result, chemical pretreatment processes currently occur in separate reaction vessels from cellulose degradation, and must occur prior to cellulose degradation.
Thus, methods that are more compatible with the cellulose degradation process, do not require high temperatures and pressures, do not generate toxic waste products, and require less energy, are desirable. For these reasons, efficient methods are needed for biomass conversion.
Filamentous fungi are efficient producers of a large variety of enzymes, and, therefore, they are exploited already for decades for the production of enzymes at industrial scale. Numerous hydrolytic activities have been identified for hydrolysis of starch, (hemi)cellulose and inulin. For many of these enzymes industrial processes have been developed.
Based on extensive research on these carbohydrolytic enzymes, besides catalytic domains also domains involved in substrate binding have been identified. For fungal enzymes in particular, most of the lignocellulose and (hemi-)cellulose degrading enzymes are characterized by having a cellulose binding domain, denominated as CBM1 (see also www.cazy.org/fam/acc_CBM.html. Interestingly, in particular for CBM1, which is unique to fungi, proteins with completely different catalytic activities have been identified. Besides different types of (hemi)cellulases, xylanases, pectinases, esterases, chitinases and lipases amongst others also CBM-1 proteins with unknown activity have been identified. The largest gene family of this latter class is the GH61 protein/gene family. However, there is still need for further enzymes involved in lignocellulose and (hemi-)cellulose degradation.